Semiconductor films, such as silicon films, are known to be used for providing pixels for liquid crystal display devices. Such films have previously been processed (i.e., irradiated by an excimer laser and then crystallized) via excimer laser annealing (“ELA”) methods. However, the semiconductor films processed using such known ELA methods often suffer from microstructural non-uniformities, which manifest themselves in availing a non-uniform performance of thin-film transistor (“TFT”) devices fabricated on such films. The non-uniformity generally stems from the intrinsic pulse-to-pulse variations in the output energy of the excimer lasers irradiating the semiconductor films. The above-described non-uniformity could manifest itself in, for example, a noticeable difference in a brightness level of the pixels in one area of the display as compared to the brightness in other areas thereof.
Significant effort has gone into the refinement of “conventional” ELA (also known as line-beam ELA) processes in the attempt to reduce or eliminate the non-uniformity. For example, U.S. Pat. No. 5,766,989 issued to Maegawa et al., the entire disclosure of which is incorporated herein in its entirety by reference, describes the ELA methods for forming polycrystalline thin film and a method for fabricating a thin-film transistor. This publication attempts to address the problem of non-uniformity of characteristics across the substrate, and provide certain options for apparently suppressing such non-uniformities.
However, the details of the beam-shaping approach used in conventional ELA methods make it extremely difficult to reduce the non-uniformities in the semiconductor films. This is especially because the energy fluence described above may be different for each beam pulse, and thus non-uniformity may be introduced into sections of the semiconductor thin film upon irradiation, solidification and crystallization.
Techniques for fabricating large grained single crystal or polycrystalline silicon thin films using sequential lateral solidification are known in the art. For example, in U.S. Pat. No. 6,322,625 issued to Im and U.S. patent application Ser. No. 09/390,537, the entire disclosures of which are incorporated herein by reference, and which is assigned to the common assignee of the present application, particularly advantageous apparatus and methods for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses and small-scale translation of a silicon sample to implement sequential lateral solidification have been described. In these patent documents, it has been discussed in great detail that at least portions of the semiconductor film on a substrate are irradiated with a suitable radiation pulse to completely melt such portions of the film throughout their thickness. In this manner, when the molten semiconductor material solidifies, a crystalline structure grows into the solidifying portions from selected areas of the semiconductor film which did not undergo a complete melting. This publication mentions that the small grain growth in regions in which nucleation may occur. As is known in the art, such nucleation generates small grained material in the area of the nucleation.
As was previously known to those having ordinary skill in the art of a sequential lateral solidification (“SLS”) as described in U.S. Pat. No. 6,322,625, which utilizes the irradiation of a particular area using beam pulses whose cross-sectional areas are large, it is possible for the nucleation to occur in such areas before a lateral crystal growth is effectuated in such area. This was generally thought to be undesirable, and thus the placement of the TFT devices within these area was avoided.
While certain TFT devices do not require a high performance level, they require good uniformity in certain applications. Accordingly, it may be preferable to generate substrates which include the semiconductor films that allow uniform small-grained material to be produced therein, without the need for a multiple irradiation of the same area on the semiconductor thin film.